Below-Ground Strength, Above-Ground Power

Sept. 28, 2010

The promises of green power prompt many positive views about energy independence and environmental favorability, but often lost in the promotion is how green machines must depend on the energy-intensive processes of turning raw materials into finished construction products.

The promises of green power prompt many positive views about energy independence and environmental favorability, but often lost in the promotion is how green machines must depend on the energy-intensive processes of turning raw materials into finished construction products.

A case in point is New York's Wethersfield Wind Farm, an 84-turbine project about 50 miles southeast of Buffalo. Spread across a mix of farmland and wooded areas, the site had natural soil that was largely suitable to support the heavy turbines, although at more than 400,000 pounds (415 kips) each they presented the potential for differential settlement in areas where soil variation was a real problem. Add to this the large overturning force (25,747 kips per foot) caused by the wind loading on these 213-foot-high structures, and the importance of having uniform foundation support became paramount to ensuring safe operation.

Atlantic Testing Laboratories, Ltd. of Syracuse, NY, evaluated the subsurface conditions as part of its geotechnical research and called for ground improvement specialist GeoStructures to determine the best solution for reinforcing the soil. The traditional approach of improving foundation conditions through over-excavation and replacement with granular fill is sound, but it gets costly if there is more than a relatively small amount of poor soil to be removed. Compounding the cost problem for a renewable energy project is the less-than-green perception of multiple truck trips needed for transporting fill material. Similarly, deep foundations that use steel, timber or drilled concrete shafts as support often require materials that consume a lot of energy during their manufacture, have to be shipped to the site, and require a lengthy installation process.

Borrowing a solution it has used on other commercial and industrial sites, GeoStructures recommended installation of an Intermediate Foundation® solution using Rammed Aggregate Pier® (RAP) technology. Construction begins with use of an auger to create a cavity, followed by placement of layers of aggregate in thin, one-foot lifts. Vertical impact ramming energy compacts the material with a patented, beveled tamper, which densifies the aggregate laterally into the cavity walls. The result is excellent coupling with surrounding soils, reliable settlement control, and enough stiffness to support many types of multi-story structures.

Even with information revealed from multiple test borings, subsurface conditions are often the riskiest part of a project because soils can vary from hole to hole, and the Wethersfield site was no different. The geology there showed ground water only 11 feet below the surface, and below that were soils that consisted of a loose sand that would easily cave in when drilled.

Faced with this, GeoStructures put forth a particular RAP system called the Impact® System. Instead of drilling a cavity and extracting soil, the Impact System drives into the ground a specially designed mandrel and tamper foot that uses a large static force augmented by dynamic vertical impact energy. These RAP elements are well suited to the loose soils at Wethersfield, and there is no need to extract, transport and dispose of spoils because all the dirt is displaced.

As the tamper foot and mandrel are forced into the ground, soil is prevented from collapsing back into the top of the mandrel because of a sacrificial steel plate that gets left at the bottom of the pier when the tamper is raised. At this point the hollow mandrel is used as a conduit for placement of the first layer of aggregate, and this gets compacted by raising the tamper foot 3 feet and driving it back down 2 feet to form a dense, one-foot lift. The hammer on the end of the mandrel provides the vertical force while the beveled tamper pushes the material into the sidewalls. Successive layers of aggregate placement and compaction create the stiffened pier and provide the necessary settlement control.

At Wethersfield a total of 306 of these RAP elements were installed under the footprint of two turbines to an average depth of 20 feet. Modulus tests performed on sample RAP elements at each location showed that they improved soil conditions enough to support the design load of 2.1 kips/square foot under normal conditions, and up to 3.8 kips/square foot under gusty conditions.

Another benefit of the Impact System is its relatively fast installation. With no spoil disposal and local sourcing of aggregate, the two-week construction period was nine days ahead of schedule. This allowed the general contractor to maintain the timetable for testing, inspection of generators, hydraulics and gears, and launch by the end of 2008.

For site owner Noble Environmental Power of Bliss, NY, which has 3,850 megawatts of wind parks under development in eight states, Wethersfield is an important part of its portfolio that will meet the growing demand for clean, renewable sources of energy in an industry that increasingly calls for more diversity in power generation. For New York, the project is a large step toward its goal of increasing to 25 percent the amount of renewable electricity that is sold in the state (currently at 19 percent). Equally important are the benefits to the businesses and residents of the region who may see more stable electricity prices.

Editor's note: Chuck Lacey Jr., P.E., is director of Business Development for GeoStructures.